1. Field of the Invention
The present invention relates to imaging systems. More specifically, the present invention relates to radar imaging systems.
2. Description of the Related Art
Imaging techniques are well known and widely used in the art. Certain imaging technologies are better suited for particular applications. For example, radar imagery is widely used for surveillance and reconnaissance as well as target tracking and identification. For radar and other imaging technologies, the ability to clearly resolve and discriminate targets may be essential in meeting objectives specified for a particular application.
One such application involves xe2x80x98real beam ground mappingxe2x80x99. Real beam ground mapping involves scanning an area, e.g., the earth""s surface, using a scanning antenna or an electronically scanned antenna. Returns from an illumination of the surface are then examined for xe2x80x98back-scatterxe2x80x99 or reflections therefrom. As the beam is scanned in azimuth, information is collected with respect to the range direction. At each beam position, the distance of various scatterers may be ascertained for each range cell. This information may then be displayed in a real beam ground mapped image.
Unfortunately, at large side-looking (azimuthal) scan angles relative to the velocity vector, the Doppler spectrum of the clutter spreads out significantly over many Doppler filters relative to the Doppler spectrum of signals received closer to the direction of the velocity vector of the platform over the entire pulse repetition frequency (PRF). In a Real Beam Ground Mapping application, this may lead to the creation of maps with poor image quality.
While range data may be resolved with adequate resolution, currently, resolution of azimuth data with comparable resolution has proved to be problematic. This is due to the fact that azimuth resolution is limited to the width of the beam and degrades as a function of range. Accordingly, the poor resolution of conventional real beam mapping systems limits the ability of the system to discriminate scatterers.
SAR (synthetic aperture radar) has been used for ground mapping. However, currently, SARs require several seconds at each beam position and are therefore too slow for many more demanding (e.g.,military) applications.
xe2x80x9cSuper resolutionxe2x80x9d techniques are widely used to sharpen the radar imagery. However, the quality achieved is scene dependent and is not robust. The current techniques do not effectively account for the impact of the radar system on the true scene.
Hence, a need remained in the art for an improved system or method for providing ground mapped images. Specifically, a need remained in the art for a system or method for providing enhanced cross-range (azimuthal) resolution for a real beam ground mapping radar system. This need was met by copending application entitled RADAR IMAGING SYSTEM AND METHOD, filed Jan. 16, 2002 by Kapriel V. Krikorian and Robert A. Rosen, Ser. No. 10/050,296 (hereinafter the xe2x80x9cKrikorian et al systemxe2x80x9d) the teachings of which are incorporated herein by reference.
Notwithstanding the fact that Krikorian et al substantially addressed the need for a system for providing enhanced cross-range resolution, an additional problem remains with respect to range and Doppler ambiguity. That is, when a conventional radar system searches in the cross-scan direction for unambiguous returns within the range of, say, 60 miles, the pulse repetition frequency (PRF) must be low. That is, the pulse repetition interval (PRI), which is equal to the pulse width plus the desired range, must be greater than 60 miles to cover the entire distance without ambiguity. Thus, inasmuch as the PRF is the inverse of the PRI, with a long PRI, the PRF must be low. If not, the Doppler spectrum becomes ambiguous and scatter returns from spurious sources of reflection begin to fall in the same range bins on top of each other. This requires processing of the returns from each pulse separately and precludes a desirable coherent integration of same. The only current option then is to integrate the pulses non-coherently by simply summing the magnitudes thereof. Unfortunately, the resulting real beam ground maps reflect the loss of sensitivity associated with this approach.
Hence, a need remains in the art for an improved system or method for providing real beam ground mapped images. Specifically, a need remains in the art for a system or method for long range real beam ground mapping with improved sensitivity at high azimuth look angles.
The need in the art is addressed by the system and data processing methods of the present invention. There are at least two significant aspects of the invention. One is the provision of a method for exciting an antenna with a waveform having a burst width and pulse width scaled proportionately with a selected range scale. The second is the provision of a temporal filter to address any ambiguities in range resulting from the transmission of a signal in accordance with the novel waveform.
The inventive filtering method includes the step of scanning a beam including a plurality of pulses of electromagnetic energy. Reflections of these pulses are received as return signals. The returns are processed to extract range in range rate measurements. The range and range rate measurements are compressed to form a plurality of range bins. The pulses are selectively weighted to reduce sidelobes resulting from a subsequent Fast Fourier transform (FFT) operation. The FFT operation is then performed for a predetermined number of pulses in at least one of the range bins at least one frequency. A second FFT operation is then performed for pixels of azimuth data across the range bins. Finally, ambiguity nulling weights are provided and applied to each pixel of data in each range bin.
In illustrative embodiments, the step of scanning the beam includes the step of outputting a beam excited by a waveform having a burst width and pulse width scaled proportionately with a selected range scale. The step of selecting and weighting return pulses in the range bins includes the step of selecting and weighting the return pulses to reduce sidelobes resulting from the Fast Fourier transform of the predetermined number of pulses. The step of selecting and weighting return pulses in the range bins includes the step of selecting the pulses based on antenna scan weight and the pulse repetition frequency of the pulses.
Further, in the illustrative embodiment, the step of performing a Fast Fourier Transform for pixels of azimuth data across the range bins includes the steps of selecting Fast Fourier Transform weighting windows and performing a fast Fourier transform for pixels of azimuth data across the range bins based on scan geometry and history of the scan beam.
In the illustrative embodiment, the step of applying nulling weights to each pixel of data in each range bin includes the step of applying nulling weights to each pixel of data in each range bin based on beam scan geometry, scan history, range, range ambiguity, and PRF Doppler ambiguity.
Further, in the illustrative embodiment, the step of applying nulling weights to each pixel of data in each range bin further includes the step of applying nulling weights to each pixel of data in each range bin at each of a plurality of predetermined frequencies. The illustrative embodiment of the inventive method further includes the step of performing pulse detection integration across each of the frequencies of the beam.
The inventive system may be implemented in software running on a processor. In a specific implementation, the system generates a novel radar waveform which effects a higher duty factor and provides better sensitivity. An additional novel aspect of the invention is the provision of a temporal filter in the azimuth direction to significantly reduce Doppler ambiguity.